This invention relates to a fast acting solid state AC circuit breaker.
Circuit breakers are normally used to protect power distribution systems by monitoring the current to a load. When the current reaches an excessive level which could cause damage to the line, the breaker opens to prevent the flow of the current to thereby protect both the line and the load. Prior art circuit breakers may comprise a fuse link which overheats as a result of excessive current and melts thus opening, or may take the form of a mechanical device which when set provides a current path, wherein the path is mechanically opened by a reaction of the breaker to a fault. Fuse link or mechanical circuit breakers, because of their inherent lag in responding to line faults, do not immediately open a circuit in response to a fault. This is acceptable, however, since the line itself can safely handle a fault for a brief period of time.
A problem exists, however, in the case of power distribution systems which are supplied by solid state frequency converters. Such converters are designed to go into a self-protecting current limiting mode if a fault of a sufficient magnitude occurs. Because of the slow reaction time of mechanical circuit breakers, such breakers do not have a chance to open before the frequency converters enter the self-protecting current limiting mode. Once this has occurred, the entire system is without power, and it is impossible to tell where the fault is since no circuit breaker has tripped to indicate the location of the fault. In such situations, the fault may be found by opening the circuit breakers one at a time beginning with those breakers which are nearest to the frequency converter until the fault is isolated. This process is very time consuming and is unacceptable in certain situations.
It would therefore be desirable to provide a fast acting circuit breaker which would open before a solid state frequency converter could switch into its current limiting mode so that a load fault would not result in loss of power to the entire system.